Historically such screens have been constructed from sheets of woven wire mesh stretched over and secured to metal frames using a polymer adhesive. Typically the frames are generally rectangular and define one or more rectangular openings over which the wire mesh is stretched.
Usually two or more layers of wire mesh having different mesh sizes have been secured to each metal frame. The tensions in the warp and weft wires of one mesh are normally greater than the corresponding warp and weft wire tensions in the other mesh.
Forms of Frame
Such constructions tended to result in relatively heavy screens and since they are typically man-handled into position a new design of frame was introduced some years ago by the Applicant Company. This was constructed largely from a GRP polymer moulding in which a wire-frame is embodied during the moulding process, to reinforce the final structure and introduce sufficient rigidity to not only contain and preserve the tensions in the wire meshes, but also to ensure that the frames did not bend under the weight of the relatively dense slurry making up the drilling mud and the build-up of solids on the screen in use.
This design of screen was ideally suited to shakers such as the VSM range of shakers supplied by Rig Technology Ltd. of Aberdeen, Scotland, UK.
The throughput of a shaker screen is dictated at least in part by the area of the screen mesh onto which the drilling mud is deposited in use. Since the area of each rectangular frame was dictated in part by the maximum permitted weight of the final screen, filtering areas greater than that of a single screen were created by arranging two or four screens in edge to edge abutment in a rectilinear rigid basket, having edge supports on which edges of the screens rested. The screens were held in place by clamps and preferably an inflatable clamping mechanism was employed to clamp the edge of the screens onto the edge supports of the rigid basket. The inflatable clamping also ensured a good liquid-tight seal around the edges of the screens.
Other shakers have been developed which accommodate large area but less well supported screens, and it has been proposed to construct such screens using wire-frame reinforced GRP frames, but after testing prototypes they were found not to have sufficient stiffness to perform in the field.
In particular the larger area GRP wire-frame reinforced screens were observed to whip violently around the centre of the unsupported span. This resulted in the screen becoming separated from edge supports to which it should remain sealed at all times in use. This allowed slurry to bypass the screen and drop into the sump reserved for filtered liquids.
In addition the whipping of the edge regions of the screen onto the edge supports resulted in damage to the underside of the screen frame.
Furthermore, excessive whipping caused considerable splashing of slurry over the walls of the basket and onto the floor on which the shaker was mounted. Quite apart from loss of relatively expensive drilling muds, the chemicals making up the muds are not such as should be dumped at sea. Therefore any such splashing could result in environmental contamination and serious penalties for rig-operators if any such spillages are not collected and disposed of correctly, all of which increased the cost of processing and recovering the down-hole mud.
It has been proposed to construct a reinforcing structure for such a screen in which the wires extend between and are secured to the upper and lower faces of a rectangular sub frame made from lengths of metal box-section, so as to increase the rigidity of the edges of the screen. However this arrangement is not suitable for all designs of screens, especially those in which the depth of the sub-frame is restricted so that the spacing between the reinforcing wires is in turn reduced, thereby reducing the bending movement of the ribs.
It is therefore an object of the present invention to provide an improved form of relatively light-weight frame construction which is sufficiently rigid as not to whip excessively in use and can span larger screening areas than the previously produced wire reinforced GRP framed screens, but can also be employed in shakers in which the depth of the screen will not allow the ribs to be reliably reinforced by pairs of spaced apart parallel wires joined at their ends to a bounding sub-frame.